Syscalls enable communication between processes, as well as communication to the kernel. These are guaranteed to never change, but new syscalls may be added.
Syscalls may take up to seven `usize` -bit arguments, and may return up to seven `usize` -bit output operands, plus a tag indicating success or failure. The size of `usize` may vary depending on processor type, and is always the width of a pointer.
Note that this means that there is a hard limit on the number of arguments that can be passed. Additionally, the RISC-V calling convention specifies that only `$a0` and `$a1` may be used to return values. Xous expands this to allow eight return values, which currently requires an assembly shim.
When built for Rust's `std` library, syscalls are sent via a network connection. Because pointers are unsafe to send, `usize` is defined on `std` as being 32-bits. Additionally, most syscalls will return `NotImplemented`.
When returning, a memory buffer may be required. The contents of this buffer will be appended to the network packet in the same manner as the calling buffer.
Syscalls use specialised types, many of which are backed by `usize`. For example, a `MemoryAddress` is a `NoneZeroUsize`, which is the same size as `usize`. In this manner, programs can ensure that memory addresses cannot be `NULL`.
System calls are all tagged enums. Syscalls may not be made from within an interrupt context, unless the name ends in `I`, for example `ReturnToParentI`.
